This invention relates to a method of cleaning a surface to be subsequently coated, such as the front face of a thermal ink jet printing device. More particularly, this invention relates to a method of cleaning a substrate so as to improve the adhesion between a substrate and a carbon layer or a fluoropolymer layer in fluoropolymer deposition processes.
Amorphous or diamond-like carbon films are known in the art. Reference is made, for example, to U.S. Pat. No. 5,073,785, which is incorporated by reference herein in its entirety. These films are quite useful for many applications, including coating applications, because of their mechanical properties, extended wearability and phobic or shunning properties for many dye and water-based ink formulations.
Fluoropolymer films are also useful for many coating applications because of their ink repellant properties. Typically, an underlayer, such as an amorphous or diamond-like carbon layer, is used between a substrate and the fluoropolymer film to promote adhesion and improve durability.
U.S. Pat. No. 4,601,777 to Hawkins et al. discloses several fabricating processes for ink jet printheads, each printhead being composed of two parts aligned and bonded together. One part is substantially a flat heater plate substrate which contains on the surface thereof a linear array of heating elements and addressing electrodes, and the second part is a channel plate substrate having at least one recess anisotropically etched therein to serve as an ink supply manifold when the two parts are bonded together. A linear array of parallel grooves is formed in the second part, so that one set of ends of the grooves communicates with the manifold recess and the other ends are open for use as ink droplet expelling nozzles. Many printheads can be simultaneously made by producing a plurality of sets of heating element arrays with their addressing electrodes on, for example, a silicon wafer and by placing alignment marks thereon at predetermined locations. A corresponding plurality of sets of channels and associated manifolds is produced in a second silicon wafer and, in one embodiment, alignment openings are etched thereon at predetermined locations. The two wafers are aligned via the alignment openings and alignment marks and then bonded together and diced into many separate printheads.
In existing thermal ink jet printing, the printhead comprises one or more ink filled channels, such as disclosed in U.S. Pat. No. 4,463,359 to Ayata et al., communicating with a relatively small ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in the channels near the nozzles a predetermined distance therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
The specific details of the separation of the ink from its physical surroundings, the ink channel and its orifice, determine to a large extent the direction in which the ink will travel to the paper and thus determine where the mark on the paper will be made. Any microscopic irregularity that would affect the isotropy of this ink-orifice separation process will usually cause the ink to travel in an uncontrolled and not intended direction, that is, for example, not orthogonal to the plane defined by the orifice. This results in poor quality of the images and text that are printed on the paper. Such irregularities include pools of ink which collect around the orifice from previous jet firing. Microscopic irregularities of the orifice can be avoided by providing a coating on the exit orifice that repels the ink that is used for the printing process. To avoid or minimize ink drop deflection problems which can lead to subsequently printed images of poor quality, ink jet head components should be coated with one or more ink repellant layers. Examples of such ink repellant coatings are disclosed, for example, in U.S. Pat. No. 5,073,785, which was previously incorporated by reference herein in its entirety. This patent discloses processes for minimizing or avoiding ink drop deflection in ink jet devices which comprise coating the front face of ink jet head components with amorphous carbon, hydrogenated amorphous carbon, halogenated amorphous carbon, fluorinated amorphous carbon or mixtures thereof, and the like.
Fluoropolymer thin films deposited by plasma enhanced chemical vapor deposition are also ink repellant and can be used as front face coatings for thermal ink jet die modules. An underlayer made of a material such as amorphous or diamond-like carbon may be used between a substrate and the fluoropolymer film to promote adhesion between these layers and to improve front face durability, although the amorphous carbon layer is not always necessary.
The adhesion between the front face substrate and the carbon layer or fluoropolymer layer is important to the performance of the ink jet device. Cleaning or "descumming" steps are typically carried out to remove contaminants originating from the dicing operation from the substrate. Otherwise, such contaminants might cause adhesion failure of the carbon layer. These include: blade resins; epoxy materials used to bond the heater and channel wafer; and organic polymer materials such as polyimide which are coated on the heater wafer to form wells within the channels in which the ink resides until pulsing is required. Cleaning processes may use a reactive plasma chemical reaction wherein the contaminants form a volatile compound which is then pumped from the deposition chamber. For example, an oxidizing gas such as oxygen or nitrous oxide has been used to remove polyimide debris caused by dicing from the front face of thermal ink jet dies.
However, cleaning processes using an oxidizing gas such as oxygen or nitrous oxide are not suitable when fluoropolymer coating processes are used. For reasons of cost, convenience, material robustness and simplicity, it is preferred that all steps in the front face coating process be carried out in a single chamber plasma processing system. However, fluoropolymer deposits accumulate and are difficult to totally remove from the internal surfaces of the plasma processing chamber such as on the walls and the electrodes. A nitrous oxide or oxygen gas or other oxidizing plasma etches the fluoropolymer deposits and causes them to redeposit uncontrollably on the substrates. This results in adhesion failure of the amorphous carbon intermediary layer or the controlled fluoropolymer front face coating layer itself if no intermediary layer is used. Etching of the fluoropolymer deposits releases radical fluorine and fluorine-containing gases, e.g., CF.sub.4, into the deposition system. These fluorine-containing species can adsorb on internal chamber surfaces and are generally difficult to remove from the deposition system by simple vacuum pumping techniques even under ultra high vacuum conditions. If the amorphous carbon underlayer is deposited while these fluorine-containing species are present, the result is a contaminant deposit, which typically has poor adhesion to the substrate, at the interface. This has been demonstrated during front face coating applications on thermal ink jet dies and can be a limiting factor for the development of improved front face coatings. Multiple chamber deposition systems can be used to address the problem by preparing the substrate (i.e., cleaning and carbon layer deposition) in one chamber and applying the fluoropolymer film in a second, thereby isolating the contamination source. However, these systems are expensive, occupy additional space and are more prone to mechanical failure than single chamber systems. In addition, the presence of oxygen atoms generated in the plasma and fluorine-containing species etched off the internal surfaces results in uncontrolled etching of the polyimide and crystalline silicon around the nozzle faces. This distorts the nozzles and causes drop misdirection and subsequent print quality defects.
Japanese Patent Document 58-190,898 discloses the use of a hydrogen plasma to remove adsorbed gases on a substrate prior to molecular beam epitaxy deposition. An article by Y. Saito amd A. Yoshida (J. Electrochem. Soc., Vol. 139, No.12, December 1992) describes a method of removing adsorbed fluorine from the surface of substrates using hydrogen gas at an elevated substrate temperature. The article does not describe the use of plasma nor fluoropolymer film deposition. U.S. Pat. No. 5,043,747 discloses the use of a front face coating formed by a wet coating of an aromatic fluoropolymer which is subsequently cured. Plasma processing is not used, nor is the use of a substrate cleaning process disclosed.